A vehicle may contain several types of safety restraint systems that are activated responsive to a vehicle crash or are employed prior to a vehicle crash for purposes of mitigating occupant injury. Examples of such restraint systems include air bags, seat belt systems including seat belt pretensioners, and deployable knee bolsters.
Since the object of these safety restraint systems is to mitigate occupant injury in the event of a collision, it is desirable that they operate reliably. For this reason, it is generally desirable to be able to sense the operability of vehicle safety systems prior to a collision so that in the event that such a system becomes inoperative corrective action may be taken.
Air bag inflators are designed with a given restraint capacity, as for example, the capacity to protect an unbelted normally seated fiftieth percentile occupant when subjected to a 30 MPH barrier equivalent crash, which results in associated energy and power levels which can be injurious to out-of-position occupants. While relatively infrequent, cases of injury or death caused by air bag inflators in crashes for which the occupants would have otherwise survived relatively unharmed have provided the impetus to reduce or eliminate the potential for air bag inflators to injure the occupants which they are intended to protect.
One technique for mitigating injury to occupants by the air bag inflator is to reduce the power and energy levels of the associated air bag inflator, for example by reducing the amount of gas generant in the air bag inflator, or the inflation rate thereof. This reduces the risk of harm to occupants by the air bag inflator while simultaneously reducing the restraint capacity of the air bag inflator, which places occupants at greater risk for injury when exposed to higher severity crashes.
Generally, occupants who are unbelted or improperly belted are at greater risk of injury in a crash than are properly belted occupants, whether or not the occupant is protected by an air bag inflator, even for relatively mild collisions.
Known mechanisms for sensing the latching of a seat belt buckle to a seat belt latch use a seat belt latch assembly employing one or more mechanical switches to sense proper closure of the latch. However, such mechanical switches are expensive, unreliable, and typically can not be diagnosed by a processor. A mechanical switch employing a set of switch contacts to open or close a circuit thereby indicating the seat belt latch state to a processor operatively connected thereto does not have the ability to inform the processor of whether the switch has malfunctioned in an open or closed state. For example, a set of mechanical switch contacts stuck in an open state appears to the processor as an unlatched seat belt, i.e., an open circuit condition. Conversely, a set of mechanical switch contacts stuck in a closed state appears to the processor as a latched seat belt, i.e., a closed or short circuit condition.